1. Field of the Invention
The present invention generally relates to a camera module for a motor vehicle, comprising a lens objective, a lens holder holding said lens objective, and a back plate connected to said lens holder and holding an image sensor in or close to an image plane of the lens objective. The invention also relates to a method of mounting a camera module for a motor vehicle.
2. Description of Related Art
In the assembly process of camera modules, the lens objective is usually aligned relative to the image sensor in order to compensate for manufacturing tolerances of the lens objective and tilt or misalignment of the image sensor, which would otherwise lead to a deterioration of the image quality. The aim of the alignment is to have the sensitive surface of the image sensor positioned in the image plane of the lens objective and to have the image centered on the image sensor. The alignment procedure usually involves adjusting five degrees of freedom, in particular bore sight correction (xy), tip-tilt or yaw-pitch adjustment (Θx,Θy) and focus adjustment (z), or six degrees of freedom, with additional roll adjustment (Θz), in particular for stereo configurations involving a plurality of co-operating camera modules.
A preferred alignment process called active alignment uses image data acquired by the image sensor of the camera during assembly as feedback for adaptively determining the optimum position of the lens objective relative to the image sensor. The lens objective is freely moved relative to the image sensor while simultaneously capturing images and thus determining the orientation of the lens objective that yields the best image quality. Once the optimal position and orientation of the lens objective has been determined, it is conventionally fixed relative to the image sensor by a glue joint between the lens objective and the front of the lens holder.
Although the optical axis approximately remains centered on the image sensor and bore sight errors are suppressed, a disadvantage of this concept is that changes of the glue due to temperature changes can result in significant shifts of the image plane. In the case of stereo systems this will lead to a violation of the stereo stability. Furthermore, since the lens objective is usually rotationally symmetrical around the optical axis, a roll angle displacement of the image sensor cannot be corrected by rotating the lens objective relative to the image sensor in the alignment process. Therefore, in a stereo system the roll alignment is performed by rotating one camera module around its optical axis during mounting of the camera modules into an external camera housing part, which is very laborious.
Also, in a stereo camera system it is required that the two cameras eyes have a coincident bore sight, with minimal change over temperature. The active alignment equipment would have to be modified significantly in order to handle both mono and stereo camera systems. It is desired to share common production processes as much as possible in order to gain efficiency and throughput, hence a modular approach for the camera eyes. It is also critical to maintain focus over the temperature envelope for any automotive camera.
DE 10 2010 047 106 A1 and WO 2011/131164 A1 respectively disclose a camera system where the lens objective is fixed relative to the image sensor by a glue joint between the lens objective and the lens holder. Here as well as displacement of the image sensor in the roll angle cannot be corrected by rotating the lens holder relative to the image sensor in the alignment process.